Comparison of Methods for the Calculation of Mass Fraction Burnt from Engine Pressure—Time Diagrams

Calculations of the mass fraction burnt as a function of time in a spark ignition engine have been made by two methods. The simple method assumes that the mass fraction burnt is in proportion to the fraction of total pressure rise due to combustion. The complex method assumes a two-zone combustion model, and computes the mass fraction burnt that is required to give the observed pressure rise for each 1° crank angle increment. The complex model allows for dissociation, variable gas properties and heat transfer from the burnt and unburnt gases. Notwithstanding the substantial differences between the two models, the results are in surprisingly close agreement. This can be attributed to the fact that the temperature of the burnt gas remains nearly constant during combustion.